BOSTON — A proposed constitutional amendment calling for what's been dubbed the "millionaire tax" has cleared a key legislative hurdle.

The House and Senate voted 135-57 Wednesday in favor of advancing the 4 percent surtax.

Under the proposal, individuals with annual incomes above $1 million would be subject to the tax. The higher tax rate would only apply to that portion of income over $1 million. The current income tax rate is 5.1 percent.